248 BYZANTINE CHURCHES CHAP, xvi each bay a door conducts into the church, the central door being set In a marble frame and flanked by two Corinthian columns, which support a bold wall arcade. The drum of the dome is a polygon of twelve sides, and was lighted by the same number of windows. It rests on four columns, which were originally square, but now have large champs at the angles, dying out at top and bottom. Barrel vaults cover the arms of the cross, and dome vaults surmount the chambers at Its angles. As in the Pantokrator (p. 235)3 ^e Astern arm is pierced by two windows in the vaulting surface. The central apse is lighted by a triple window, having oblong shafts, circular on their inner and outer faces, and bearing capitals now badly injured. A niche indents the northern, eastern, and southern interior walls of the apsidal chapels. The windows in the northern and southern walls of the church have been built up almost to their full height, leaving only small openings for light at the top. There can be little doubt that they were triple windows with a parapet of carved marble slabs between the shafts. On the exterior the apse shows five sides, and is decorated by an arcade of five arches and an upper tier of five niches. The lateral apses do not project beyond the face of the eastern wall, but. are slightly marked out by cutting back the sides and forming angular grooves. Bayet* assigns the church to the ninth or tenth century, the age of Leo the Wise and Constantine Porphyrogenitus. Fergusson2 is of the same opinion so far as the earlier portions of the building are concerned. But that date is based on the mistaken view that the building is the church of the Theotokos erected by Constantine Lips. Diehl3 assigns the church to the second half of the eleventh century. In the library of the Royal Institute of British Architects, in London, are four volumes of Texier's sketches and drawings of buildings in or near Constantinople. In that collection is found a complete set of drawings of this church, showing a chapel on both the north and south sides of the building, and even giving measurements on the south 1 U Art by&antln, p. 126. 2 History of Architecture, vol. i. p. 4.58. 3 Manuel d'art byzantin, p. 414.